iPhone 6S 4G connectivity fails

[P Lawrence]

My wife seems unable to search the internet away from home. The 4G connectivity setting is set on, but the message 'offline' appears instead of internet access. However, I have no such difficulty with my Samsung phone.

Therefore, suggestions / directions to help resolve this would be appreciated.

Software version is 15.7.5.

N.B. This has been occurring consistently for some months.

Thank you in advance.